1. Technical Field
This disclosure generally relates to a liquid jet device, an image forming apparatus, and a method for adjusting landing positions of liquid droplets.
2. Description of the Related Art
There are image forming apparatuses (e.g., a printer, a fax machine, a copier, and a multifunction copier having functions of a printer, a fax machine, and a copier) that use a liquid-jet device including a recording head implemented by a liquid-jet head to form (record or print) an image on paper (not limited to a sheet of paper but also refers to any medium on which an image can be formed, and may also be called a recording medium, recording paper, recording sheet, recording material, etc.). A liquid-jet device jets droplets of a recording liquid (or ink) from a liquid-jet head onto paper being carried in an image forming apparatus and thereby forms an image on the paper.
In the present application, an image forming apparatus refers to an apparatus that forms an image by jetting a liquid onto a recording medium made of paper, thread, fabric, silk, leather, metal, plastic, glass, wood, ceramic, etc. Also, “image forming” indicates not only a process of forming an image such as a character or a figure having a meaning on a recording medium, but also a process of forming a meaningless image such as a pattern on a recording medium. In other words, an image forming apparatus may even refer to a textile printer or an apparatus for forming a metal wiring pattern. Liquids used in an image forming apparatus are not limited to a recording liquid and ink. Further, a liquid-jet device refers to any device that jets a liquid from its liquid-jet head. The use of a liquid-jet device is not limited to image forming.
In a liquid-jet device or an image forming apparatus, a carriage having a recording head is moved forward (forward scan) and backward (backward scan) and recording (or printing) is performed in both the forward and backward directions (bidirectional printing). When printing lines with such a liquid-jet device or an image forming apparatus, misalignment tends to occur between lines printed by the forward and backward scans.
To solve this problem, some inkjet recording apparatuses have a line-adjustment function for adjusting the positions of lines. With a line-adjustment function, for example, the user prints a test chart and enters an adjustment value based on the results on the printed test chart to adjust the timing of jetting ink. However, selection of the adjustment value varies between users and depends on the ability of the user. If an incorrect adjustment value is entered, it may worsen the problem.
Patent document 1 discloses a liquid-jet image forming apparatus having a function to correct image density irregularity. In the disclosed image forming apparatus, a test pattern is printed on a recording medium or a conveyor belt, color data of the test pattern are obtained by scanning, and drive conditions for a recording head are adjusted based on the obtained color data to correct image density irregularity.    [Patent document 1] Japanese Patent Application Publication No. 4-39041
Patent document 2 discloses an inkjet recording apparatus capable of detecting a defective nozzle of a liquid-jet head. In the disclosed inkjet recording apparatus, a test pattern composed of dots of different colors is formed using a cyan ink, a magenta ink, and a yellow ink in an area on a recording medium conveying part, the test pattern is scanned by an RGB sensor, and a defective nozzle is determined based on the scanned test pattern.    [Patent document 2] Japanese Patent No. 3838251
Patent document 3 discloses an inkjet recording apparatus having a calibration function. In the disclosed inkjet recording apparatus, a test pattern composed of one or more of a nozzle clogging detection pattern for detecting nozzle clogging, a color shift detection pattern for detecting a color shift, and a head position adjustment pattern for adjusting the position of a recording head is formed on a part of a conveyor belt, the formed test pattern is scanned using an imaging device such as a charge-coupled device (CCD), and calibration is performed based on the scanned test pattern.    [Patent document 3] Japanese Patent Application Publication No. 2005-342899
Patent document 4 discloses an electrophotographic image forming apparatus capable of detecting the density of toner images formed on a photosensitive drum using a sensor. The sensor includes a light-emitting element for illuminating the toner images, a light-receiving element for receiving specularly reflected light from the toner images, and a light-receiving element for receiving diffusely reflected light from the toner images. With the sensor, the image forming apparatus can detect the density of toner images having different characteristics.    [Patent document 4] Japanese Patent Application Publication No. 5-249787
Patent document 5 discloses a method of determining the amount of adhering toner based on detection results from a sensor capable of detecting both specularly reflected light and diffusely reflected light from a toner image.    [Patent document 5] Japanese Patent Application Publication No. 2006-178396
According to technology disclosed in patent documents 1 through 3 described above, a test pattern is formed on a conveyor belt and the formed test pattern is scanned to obtain its color data based on which various adjustments are made. One problem with the disclosed technology is that if the color of an ink is similar to that of the conveyor belt, it becomes difficult to obtain accurate color data of a test pattern. One way to obviate this problem is to use light sources with different wavelengths corresponding to respective colors. However, this method increases the cost of a detecting unit or an imaging unit for obtaining color data of a test pattern. For example, there is a conveyor belt implemented by an electrostatic belt comprising an insulating layer on the upper side and a medium-resistance layer containing carbon for adjusting electrical conductivity on the back side. Since such a conveyor belt has a black color similar to that of a black ink, it is difficult to correctly detect a black part of a test pattern based solely on reflected light from the test pattern or by scanning the test pattern with an imaging unit.
More specifically, with the image forming apparatus disclosed in patent document 1, since a test pattern formed on a recording medium conveying part is scanned by a sensor, it is difficult to obtain accurate color data of the test pattern if the color of an ink used to form the test pattern is similar to that of the conveying part. Thus, the disclosed configuration makes it necessary to provide a filter for each color and therefore increases the production cost. In the inkjet recording apparatus disclosed in patent document 2, an RGB sensor is used to scan a test pattern formed on a recording medium conveying part. Also with this configuration, it is difficult to obtain accurate color data of the test pattern if the color of an ink used to form the test pattern is similar to that of the conveying part. Therefore, to improve the accuracy of the color data, it is necessary to limit the colors of inks used with the recording medium conveying part. Also, since a laser beam used by the RGB sensor scans extremely small spots one by one, the result of scanning tends to be affected by a tiny foreign object or a flaw on the conveying part. Further, an RGB sensor requires a light-receiving element for each color and is therefore expensive. In the inkjet recording apparatus disclosed in patent document 3, an imaging device is used to scan a test pattern formed on a recording medium conveying part. With this configuration, it is difficult to obtain accurate color data of the test pattern if the color of an ink used to form the test pattern is similar to that of the conveying part. Also, since the imaging device recognizes the test pattern as a two-dimensional image, a processing system with higher performance than that for processing a one-dimensional image is necessary. This in turn increases the cost of the inkjet recording apparatus.
To obviate the above problems, research is being conducted to apply the method disclosed in patent documents 4 and 5 for detecting the density of toner images or the amount of adhering toner to a liquid-jet image forming apparatus. Since the shape of toner particles does not change even when they are brought into contact with each other, it is possible to form a test pattern by heaping up toner in the form of a line and to accurately scan the test pattern. However, if this method is applied to a liquid-jet image forming apparatus without change, it is not possible to accurately scan a test pattern since liquid droplets clump together.
Meanwhile, a method where a test pattern is formed by jetting ink droplets onto plain paper and is scanned by an optical sensor also has a problem. With this method, bleeding caused by penetration of ink into the plain paper results in a blurred test pattern and makes it difficult to accurately detect the landing positions of ink droplets (positions of jetted ink droplets on a target surface).